A. Field
The present invention concerns a machine with an improved bearing lubrication.
B. Related Art
More particularly, the invention concerns a machine with an improved bearing lubrication, which machine mainly consists of a housing and a rotor which is provided on a shaft, provided in a rotatable manner in the above-mentioned housing by means of oil-lubricated bearings.
Several methods for lubricating such bearings are already known, such as an oil bath lubrication, an oil circulation system, an oil injection lubrication, the oil drop method, an oil mist lubrication and what is called the oil minimum lubrication.
Depending on the application, a specific lubrication method is selected, whereby it is always important to find a balance between sufficient oil being available on the one hand to build up a lubricating film which guarantees that there is no contact between the rolling elements and the bearing rings, and not more oil being admitted than is strictly necessary in order to avoid additional bearing losses on the other hand, which in turn also produce a rise in the bearing temperature.
A problem which occurs, for example in case of screw compressors, is that, due to the heat development of the compression, the rotor is heated, and that this heat of the rotor concerned, is dissipated via the rotor shaft and the bearings of this rotor shaft to the environment, as a result of which the bearings are heated.
Such a phenomenon also occurs with electric motors and generators, whereby the bearings are heated by heat losses via the rotor shaft.
Such a heating of the bearings leads to premature wear, more particularly of the roller elements and of the bearing rings of the bearings, as the warming up has a negative influence on the viscosity of the lubricating oil, and this viscosity may be insufficient to build up the lubricating film in a sufficient manner.
In order to avoid this problem, an excessive amount of oil is at present being directed to the bearing, whereby this oil serves as cooling liquid for the bearing, as a result of which the temperature of the bearing can be maintained within the required limits.
The excessive use of oil is disadvantageous however, in that there are additional, unwanted bearing losses, as the oil which is present in the bearing is turned over, and which bearing losses become in particular apparent at high rotational speeds, leading to a lower efficiency of the bearing as such, and consequently of the machine as a whole.
Especially in the case of high-speed motors, which are very compact, the phenomenon occurs, as the heat resulting from the mechanical and electric losses in the motor shaft can only be dissipated via a limited surface.
Another disadvantage of the bearings being warmed up, is that the bearing cage must usually be made of a special material which resists high temperatures, which is expensive.
When ball bearings with an angular contact are used, the problem of the bearing losses is further increased as a result of the pumping operation of this type of bearing.
GB 595.346 describes a machine in the form of a turbine or a compressor, which is provided with a bearing which is lubricated by means of an oil mist. Further GB 595.346 describes the presence of a separate cooling circuit which uses air or gas to cool the shaft of the machine.
A disadvantage of such known machines is that they are complex and require completely separate circuits for cooling the shaft and lubricating the bearing. Also, the air or gas cooling will be insufficient for cooling the shaft and for realizing a proper functioning and a long life of the machine.
From U.S. Pat. No. 6,579,078 is known a centrifugal compressor which is driven by a high-speed motor and which is bearing-mounted in a housing, whereby the lubricating oil of the slide bearings also serves to cool the bearings.
The basic idea of U.S. Pat. No. 6,579,078 is to direct cooled oil through the slide bearings in order to cool and lubricate the bearings and to moreover use the excessive oil going through the bearing to flow onto the shaft and to thus cool the latter.
The excessive lubrication of the bearings leads to additional turbulence losses, and the cooling oil is moreover heated in this bearing, as a result of which the shaft, which is irrigated by the oil, will be cooled less efficiently.
However, these additional cooling methods are not efficient and require additional measures, in this case in the form of compressed air injection, in order to avoid that oil ends up in the space between the rotor and the stator. The present invention aims to remedy one or several of the above-mentioned and other disadvantages.